Why Do Some Pestos Contain Ascorbic Acid?

Short answer
Ascorbic acid is simply another name for vitamin C, which is a natural antioxidant. Pesto makers add it to their sauces in tiny quantities to help keep the sauce looking fresh and vibrant over its long shelf life.

Long answer
There are plenty of ingredients that producers put into food that might at first sound scary. Ascorbic acid joins titanium dioxide, methylcellulose, potassium sorbate, and dozens of other ingredients that are used by sauce-makers to improve their product's taste, vibrancy, and shelf-life.

In the case of ascorbic acid, this is simply the scientific name for vitamin C. While nothing will ever completely stop herbs like basil from dulling over time, ascorbic acid has natural antioxidant properties and is one way manufacturers can help slow down the browning process.

The acid is normally added in powdered form, but only in tiny quantities. It rarely makes up any more than 0.005% of the total weight of the sauce.

Ascorbic acid doesn't feature in the official pesto recipe because its purpose is to slow the speed of oxidation, and that is unnecessary if you are planning to eat the sauce the same day. If you plan to keep your sauce for several days, then the simplest thing to do is blanch your basil in boiling water for a few seconds. This will kill the enzymes responsible for browning and keep the sauce looking vibrant green for longer.